Extensive investigations have been undertaken over many years to find materials that will be biologically and chemically stable toward body fluids. This area of research has become increasingly important with the development of various objects and articles which can be in contact with blood, such as artificial organs, vascular grafts, probes, cannulas, catheters and the like.
Synthetic plastics have come to the fore as preferred materials for such articles. However, these materials have the major drawback of being thrombogenic. Thrombogenicity has conventionally been counteracted by the use of anticoagulants such as heparin. Exemplary of procedures for attachment of heparin to otherwise thrombogenic polymeric surfaces are the disclosures in U.S. Pat. No. 4,613,517 to Williams et al. and U.S. Pat. No. 4,521,564 to Solomon et al.
In general, the most blood compatible plastics known are the fluorinated polyolefins, such as polytetrafluoroethylene, and the silicone polymers. However, while being basically hemocompatible, silicone polymers do not have the desired mechanical strength for most blood-contacting applications. One approach to improving the mechanical properties of silicone polymers has been addition of appropriate fillers and curing agents. Such additives, although providing strength, are usually themselves thrombogenic so that the improved physical strength is offset by the reduced blood compatibility.
Another approach has been to combine the blood compatibility of the silicone with the excellent mechanical properties of polyurethane. U.S. Pat. No. 3,562,352 discloses a copolymer consisting of about 90% polyurethane and 10% polydimethylsiloxane. This material, under the trade name Cardiothane.RTM. (Kontron Cardiovascular, Inc., Everett, MA), has been widely used in blood contacting applications, but has the major drawback that it is not thermoplastic and cannot be melt processed.
Thermoplastic polyoxyalkylene polyurethanes having up to 15% of a soft segment formed from a polysiloxane devoid of oxygen atoms bonded to both silicon and carbon are disclosed by Zdrahala et al. in U.S. Pat. No. 4,647,643. Polyurethanes prepared from 1,3-bis(4-hydroxybutyl) tetramethyl disiloxane are reported by Yilgor et al. in American Chemical Society Polymer Preprint 20, 286 (1982) and are suggested to have possible utility in the biomedical field.
Silicone coatings have been achieved by plasma polymerization of silicon-containing monomers onto various polymeric base materials. Preparation and hemocompatibility studies of such materials are described by Chawla in Biomaterials 2, 83 (1981).
Ward, in U.K. Patent GB No. 2,140,437B, disperses up to 5% of a silicone containing additive in a polymeric base material by mixing the components as a melt or in a solvent. Biomedical devices are prepared therefrom by conventional techniques such as injection molding and by homogeneous extrusion.
Flynn, in U.S. Pat. No. 4,581,390 discloses multiwall catheters prepared by coextrusion of a polymeric material and a composition containing a polyurethane having dispersed therein a platinum-cured silicone network polymer and a radiopaque material.
Multilayer films prepared by coextrusion are disclosed by DeAntonis et al. in U.S. Pat. No. 4,677,017.
While significant advances have been made toward blood compatible surfaces for fabrication of medical devices, further improvements are needed. In particular, materials having surfaces that are essentially non-thrombogenic for use in devices which will be in contact with blood for prolonged periods are needed. It is toward fulfillment of this need that this invention is directed.